A Phrygian ♭4 Scale
A Phrygian ♭4 contains 7 notes: A, B♭, C, D♭, E, F, G. It has 2 flats: B♭, D♭. The step pattern is H–W–H–W+H–H–W–W.
An unusual mode that looks extremely dark on paper but hides a secret: the b4 is enharmonically a major 3rd, creating an unexpected brightness lurking inside a Phrygian framework.
The formula is 1, b2, b3, b4, 5, b6, b7. The b4 is enharmonically equivalent to a major 3rd, so the scale actually contains both b3 and an enharmonic 3. This ambiguity between major and minor quality is its defining characteristic and a source of interesting interval paradoxes.
Primarily theoretical, but the hidden major 3rd can be exploited for brief moments of unexpected brightness in otherwise dark passages. Use it as a passing color in contemporary classical or avant-garde contexts.